Read the original Guru Blog post here
So what speakers are you using as monitors and what does their frequency response plot look like?
If your manufacturer has provided a plot, do they tell you how is was measured?
Of course, there is a lot more to a good speaker than a flat frequency response and even if the speaker is ruler-flat, your room probably interacts with it, and 'comb filtering' effects can easily cause +/-9 dB lumps and dips, but I'll save that discussion for a future Guru Blog.
FYI: I have attached the Frequency response plot of some B&W 805's, not the 805S shown in the pic below, but closely related (Measurements: Anechoic response on tweeter axis at 45", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz, Borrowed from Stereophile )
EDIT: Just a note if you have never looked at Speaker frequency plots before, they show frequency on the horizontal axis and output response to a signal that should be equally loud at all frequencies. That is a straight line from 20 Hz to 20 kHz, the range of human hearing. This would mean that what you put into the speaker (music) is what you get out without changes in the sound. In other words the 'ideal' speaker. Unfortunately the ideal speaker doesn't exist
Here's a site that discusses the ins and outs of these plots Understanding Speaker Frequency
Regards Scott